A so-called radial gap type rotating electrical machine includes a field which rotates on a rotation axis, and an armature which faces the field through a cylindrical gap in a direction vertical to the rotation axis. The field includes, for example, a permanent magnet and the armature face the permanent magnet through the cylindrical gap. The armature includes teeth and an armature coil which is concentrated-wound around the teeth.
To satisfy both reduction in an environment load such as global warming and realization of convenient and comfortable living environment, requirement of enhancing efficiency with respect to a rotating electrical machine, especially an electric motor further increases.
On the other hand, cost of raw materials also increases, and there is an extremely harsh reality of requirement of cost reduction of the rotating electrical machine.
To realize a compact and efficient rotating electrical machine capable of satisfying both the requirements, a rotating electrical machine using a high-performance permanent magnet is most dominant.
When such a rotating electrical machine is used under various kinds of environments, depending upon temperature characteristics of a permanent magnets, a ferrite magnet has a problem of demagnetization at low temperature, and a neodymium-iron-boron magnet has a problem of demagnetization at high temperature, for example.
In the case of the neodymium-iron-boron magnet for example, a demagnetization resistance of a magnet can be increased by enhancing coercivity by including dysprosium and terbium. However, these elements are rare and expensive.
The demagnetization resistance is enhanced also by increasing a thickness of a permanent magnet. However, use of a thick permanent magnet leads to cost increase.
Prior art documents which are associated with the present application are described as follows: Japanese Patent Application Laid-Open No. 61-94548; Japanese Patent Application Laid-Open No 11-146584; Japanese Patent Application Laid-Open No. 2002-84690; Japanese Patent Application Laid-Open No. 2002-112479; Japanese Patent Application Laid-Open No. 2003-143788; Japanese Patent Publication No. 4004894; Japanese Patent Application Laid-Open No. 2004-104962; Japanese Patent Publication No. 3737492; and Japanese Patent Publication No. 4065829. For example, Japanese Patent Application Laid-Open No. 61-94548 shows a so-called inner rotor type configuration in which a rotor is placed in the vicinity of a rotation axis. A yoke is placed closer to the rotation axis than a permanent magnet, and a hollow gap is provided in the yoke. According to Japanese Patent Application Laid-Open No. 61-94548, magnetic flux which is produced by an armature coil and which causes armature counteraction does not pass through the hollow gap and hence, a magnetic circuit which causes the armature counteraction is prevented from being produced. Magnetic flux which causes the armature counteraction acts as demagnetization field with respect to a permanent magnet.